The particular class of circuit breaker of present concern is a well-known type involving a manually operated toggle that drives a pivoted "heart" in closing the circuit breaker. The toggle becomes erect and slightly over-set to lock the "heart" in closed condition, but the heart itself incorporates overcurrent releasable latch means that allows the breaker to open despite the locked condition of the toggle. The "heart" includes three main parts: an elongated contact arm, an actuator, and an overcurrent latch device. The heart is operable as a unit about a pivot roughly midway between the ends of the contact arm. The actuator is pivoted to one end of the contact arm. The opposite end of the contact arm carries the movable contact of the circuit breaker. An elongated bimetal forming part of the overcurrent release device has one end secured to the contact arm near the movable contact. The bimetal carries a pivoted latch that normally obstructs the actuator, the heart constituting a latched unit in this condition. Deflection of the bimetal due to heating by circuit breaker current carried by the bimetal shifts the latch in the release direction. Associated with the bimetal is an electromagnet that also can operate the latch in the release direction.
Various forms of the toggle-and-"heart" type of circuit breaker are shown in the prior art, conforming to all of the above description. In the past, it has always seemed necessary to incorporate a coil with the core or yoke and the armature whenever a sensitive magnetic trip was desired. Notably, in many cases, most if not all of the structure forming the thermal and magnetic trip elements move bodily with the contact arm when the circuit breaker closes and when it is opened either manually or automatically. That basic requirement of a bodily movable thermal-magnetic trip device inherently has constraints in the way of achieving practical and economical circuit breakers of the type.